Capacitive microphones having a membrane and at least one fixed counter-electrode are known generally. In the known microphones, the membrane is prestressed by means of which the acoustic properties of the microphone capsule can be influenced. The counter-electrode is provided with channels embossed, on the one hand, for the purpose that the air can outstream into a back volume of the transducer from the air gap defined by the membrane and the counter-electrode and, on the other hand, the damping losses in the air gap are reduced. However, the sensitivity of the known microphones is lowered and the frequency response curve is unfavorably influenced. The signal conversion is effected by evaluating the relative capacitance change of the transducer.
Recent advances in semiconductor technology permit the manufacture of miniature transducers by micro-mechanical means, for example on the basis of silicon. The technical literature contains an article entitled "KAPAZITITITVE SILIZIUMSENSOREN FUER HOERSCHALLANWENDUNGEN", (translated as "Capacitive Silicon Sensors for Acoustic Application"), which appeared in 1986 in the VDI-VERLAG ISBN 3-18-146010-9, wherein the construction of a silicon microphone is described. This transducer, which has been manufactured by micro-mechanical means has the dimensions of about 1.6 mm.times.2 mm.times.0.6 mm. The active membrane surface consists of a metallic layer which is covered by a silicon nitrate layer, which is separated by an air gap from a confronting counter-electrode that is also made of silicon.
The semiconductor technology manufactured miniature microphones have some significant drawbacks, however, which are caused by damping losses in the very narrow air gaps. When the membrane is stimulated to oscillation by a periodic pressure change, a streaming resistance forms in the air gap. This streaming resistance is much higher the smaller the air gap is, since the losses in the first instance occur due to friction at the walls. The streaming resistance is, moreover, frequency dependent; it increases with increasing frequency, so that the sensitivity at higher frequencies is considerably lowered. Since the damping losses do not increase linearly with a gap narrowing, the negative influence in microphones of the aforedescribed type is particularly high. The ability to perforate the counter-electrode is not practical because of its small size and because of a technology gap which exists at present. The microphones which are described in the aforementioned literature have their sensitivity lowered as a result of the air gap losses by values under -60 dB, relative to 1 V/Pa and the frequency response is limited to several kilohertz.